1. Field of Invention
The present invention relates to paper machine forming fabrics and is particularly directed to a composite fabric comprised of at least two complete weaves, each having its own set of warp and weft yarns, with a warp or weft binder yarn that interconnects the two layers. The upper weave, that is the paper-side weave, is provided with flattened warp yarns.
In the continuous manufacture of paper, the paper machine is comprised essentially of a forming section, a press section, and a dryer section. In the forming section a dilute slurry of fibers and fillers is directed onto the surface of a moving forming fabric by means of a head box. As the forming fabric moves along the forming section, water is removed from the slurry by gravity and various dewatering devices. By the end of the forming section a continuous wet but self-supporting web of fibers and fillers remains on the surface of the forming fabric. The web then passes out of the forming section into the press section where more water is removed by mechanical pressing, after which the web passes into the dryer section where the remaining water is removed by an evaporative process.
2. Description of prior art
In recent years forming fabrics have been woven of plastic polymeric filaments in single-layer twill patterns and, although improvements have been made to produce reasonably satisfactory single-layer fabrics, the more recent development of multi-layer forming fabrics has given additional benefits to paper makers by providing increased fiber retention and fabric stability.
Typically, the paper side or upper layer of a composite forming fabric of the prior art is a fine mesh plain weave, which provides excellent retention of fibers, good dewatering, and a minimum of mark in the paper produced on its surface. The running side, or bottom layer, of such a composite fabric is usually a coarser mesh, with larger diameter strands than those of the upper layer, in order to provide resistance to stretching, narrowing, and wear.
The two layers of a composite fabric are typically interconnected in one of two ways. The first and most common method is to use a weft binder, which is usually a finer diameter yarn than those of the two layers, and is woven so as to interweave the top and bottom warp yarns and thus bind the two layers together. The other method is to interweave the warp yarns of the top layer with the weft yarns of the bottom layer, so as to bind the two layers together.
Composite forming fabrics having this description and with various binder yarn configurations are well known, examples of which are described in Canadian Pat. No. 1,115,177 and U.S. Pat. No. 4,501,303.
The importance of fabric surface geometry and, in particular, the size of the surface openings (frames) defined by the strands in the top layer, is described in the inventor's paper "Retention and Drainage of Multilayer Fabrics" (Pulp & Paper Canada, May 1986). For optimum fiber retention, it is advantageous to make these openings, particularly their machine direction lengths, as small as possible. In addition, it is often desirable to make the openings in the fabric small so that the dewatering capacity of the fabric is reduced, and thus more controlled.
One of the problems suffered by paper machine screens made as composite fabrics is that the plain weave construction of their upper layer, by the very nature of the weave geometry, imposes severe restrictions on the degree to which the size of openings in the fabric can be reduced.
Another problem suffered by composite fabrics in some applications arises from their greater thickness, which increases the void volume, resulting in higher volumes of water being carried by the fabric. On some paper machines, the greater thickness of the composite fabric results in unacceptable defects in the formation of the paper web.
A further problem suffered by composite fabrics is that the warp or weft binder yarns distort the upper paper-makaing surface, typically creating a localized surface depression often referred to as a "dimple". If the "dimple" is too deep, or results in blockage of some of the openings in the top layer, an unacceptable wire mark may be produced in the paper sheet formed on the top layer.